1. Field of the Invention
This invention relates to chemically-modified peptides having antimicrobial activity and methods of making them and using them to combat microorganisms. Chemically-modified peptides of the present invention are useful in treatment of industrial aqueous systems as well as pharmaceuticals to treat clinically relevant diseases for mammals, plants, avian and aquatic organisms, but their application is not limited thereto.
2. Background of the Invention and Related Information
Peptides are now recognized as part of a global defense mechanism used by animals and plants in terrestrial and marine environments to prevent microbial attack. The discovery of antimicrobial peptides has generated interest in the use of these compounds to combat clinically relevant microorganisms, in particular, multi-drug resistant organisms. Large screening programs have been developed to identify potential peptide-based drug candidates from both natural product-and combinatorial chemistry-derived libraries. Antimicrobial peptides are also potential candidates for the prevention of biofouling in industrial water systems, where they would represent a novel chemical class of antibiofouling compounds.
Peptides are produced naturally in bacteria, fungi, plants, insects, amphibians, crustaceans, fish and mammals [Hancock, Advances in Microbial Physiology, 135-175, Academic Press (1995)]. They represent a major inducible defense against microbes and their production in the immune system of many species is controlled by transcriptional elements. For instance, in humans, antimicrobial peptides are found in neutrophils which are responsible for responding against invasion of foreign organisms [Lehrer et al. ASM News, 56, 315-318, (1990)]. Natural antimicrobial peptides have a moderate spectrum of activity against microbes and are usually present in moderate amounts. Natural antimicrobial peptides of 12-50 amino acid residues have been obtained in the past 20 years via isolation from the defense systems of insects, amphibians and mammals [Oh et al. J. Peptide Res., 56, 41-46, (1998)]. Use of these peptides in clinical trials has shown effective antimicrobial activity [Hancock, Exp. Opin. Invest. Drugs, 7, 167-174, (1998)].
Treatment of microorganisms with antibiotics has resulted in inadequate inhibition of bacterial growth due to resistance. Peptides have shown excellent activity against antibiotic resistant microorganisms in vitro [Hancock and Lehrer, TiB Tech., 16, 82-88, (1998)].
The charge distribution and hydrophobic properties of a peptide appear to be important factors in determining its effectiveness. The peptides are usually large (12-50 amino acids) and said to be cationic due to the presence of positively charged basic amino acid residues such as arginine and lysine [Hancock, Exp. Opin. Invest. Drugs, 7, 167-174, (1998)]. It is suggested that the cationicity of the peptide may play an important role in the peptide interaction with negatively charged membranes. For instance, cationic peptides are said to compete with divalent cations on the surface of Gram-negative bacteria and prevent their interaction with lipopolysaccharide (LPS) molecules [Hancock, Exp. Opin. Invest. Drugs, 7, 167-174, (1998)]. It is hypothesized that the displacement of divalent cations by cationic peptides creates a distortion in the outer membrane of the bacteria through which peptides may pass.
Industrial facilities employ many methods of preventing biofouling of industrial water systems. Many microbial organisms are involved in biofilm formation in industrial waters. Growth of slime-producing bacteria in industrial water systems causes problems including decreased heat transfer, fouling and blockage of lines and valves, and corrosion or degradation of surfaces. Control of bacterial growth in the past has been accomplished with biocides. Many biocides and biocide formulations are known in the art. However, many of these contain components which may be environmentally deleterious or toxic, and are often resistant to breakdown.
The manufacturing cost of peptides may be a limiting factor in their antimicrobial application [Hancock and Lehrer, TiB Tech., 16, 82-88, (1998)]. The long chain length of the natural antimicrobial peptides is a major factor contributing to their cost of synthesis.
U.S. Pat. No. 5,504,190 describes a process for solid-support synthesis of equimolar oligomer mixtures that prevents unequal reaction yields during addition of blocked amino acids and allows for equal and precise representation of amino acid residues along the chain of the peptide. A hexapeptide library is described which contains 64,000,000 peptides. The peptides can be modified with a C1-C8 N-terminal acyl group. N-terminally acetylated hexa- and heptapeptides are described which are said to exhibit antimicrobial activity.
Another U.S. Pat. No. 5,512,549 discloses a peptide having 29 amino acid residues and modified with a C6-C10 acyl chain which is said to be useful in the treatment of non-insulin dependent diabetes mellitus. The peptides are not said to exhibit antimicrobial activity.
Antimicrobial activity of N-acylated derivatives of an arginine, lysine and tryptophan rich segment of lactoferricin B has been described [Wakabayashi et al, Antimicrobial Agents and Chemotherapy, 43, 1267-1269, (1999)]. Acyl chains were 6 to 10 carbons long; C-10 giving optimal activity against Escherchia coli, Pseudomonas aeruginosa and Staphylococcus aureus. 
The present invention satisfies the need in the art with short-chained peptides which are easier to produce and have effective antimicrobial activity.